During the past several years, proton conducting polymers have attracted much attention due to their considerable promise for applications in some electrochemical devices, such as displays or sensors, and which is most important, as proton exchange membranes (PEM) in PEM fuel cells (PEMFC) and direct methanol fuel cells (DMFC). In PEMFCs and DMFCs, PEMs serve as separators for the reactants, catalysts support and provide the required ionic pathway between the anode and the cathode. Therefore, their properties such as proton conductivity, water maintenance, permeability for fuel and chemical stability are crucial for the fuel cells performance. Although perfluorosulfonic acid ionomers such as Nafion®, developed by DuPont, are considered state-of-the art, their high cost, difficulty in preparation, high methanol crossover and dramatic decrease in proton conductivity at temperatures over 80° C. due to the dehydration of membranes limit their further applications. As a response to the commercial need for less expensive and more versatile polymer electrolytes, the synthesis and characterization of new membrane materials has become an active research area1.
High performance polymers are an important category of alternative candidates for PEMs. Many kinds of high performance polymers, such as poly(aryl ether sulfone)s, poly(aryl ether ketone)s, poly(ether imide)s, polybenzimidazole, poly(phenylene oxide), poly(phenylene sulfide), etc, are well known for their excellent thermal, mechanical and dielectric properties and good oxidative resistance. After modification they show rather high proton conductivities2-23 and become promising PEM materials. Wang and McGrath8 for example, reported the synthesis of biphenyl-based poly(arylene ether sulfone)s containing sulfonic acid groups by direct polymerization reactions of dipotassium 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone (SDCDPS), 4,4′-dichlorodiphenylsulfone and 4,4′-biphenol. The proton conductivity values at 30° C. of 0.11 S/cm for 40% SDCDPS copolymer and 0.17 S/cm for the 60% SDCDPS copolymer were measured. Our group17-19 and Xiao et al20, 21 also reported the synthesis and conductivities of poly(phthalazinone ether ketone)s and poly(phthalazinone ether sulfone)s containing sulfonic acid groups, prepared by both, post-synthesis sulfonation reactions and by direct polymerization reactions. Both methods gave polymers with conductivities higher than 10−2 S/cm at around SC 1.0. However, this category of polymers has a tendency to swell at high humidity and elevated temperature, especially the polymers with high sulfonic acid content. As a consequence the membranes lose the mechanical strength and their ability to function under FC conditions becomes questionable. Aromatic poly(aryl ether nitrile)s are a new class of high performance thermoplastic polymers that exhibit good mechanical properties, high chemical and thermal resistance and have already been used as matrices in advanced composites in aerospace industries24-38. Aromatic poly(aryl ether nitrile)s have been prepared by Kricheldorf, McGrath and other researchers24-38 via nucleophilic substitution polycondensation reactions of bisphenols and dihalobenzonitriles or dinitrobenzonitriles in dipolar solvents. Unlike many other poly(aryl ether)s, poly(aryl ether nitrile)s have strongly polar nitrile groups, pendant on aromatic rings, which will most probably promote adhesion of the polymers to many substrates via interaction with other polar chemical groups. It is believed that for PEM applications, the enhanced adhesive ability of aromatic poly(aryl ether nitrile)s to inorganic compounds is beneficial for adhesion of catalyst to the PEM. Recently, it was reported39,40 that nitrile groups were introduced into poly(aryl ether sulfone)s containing sulfonic acid groups with the aim to decrease the swelling of membrane films via enhanced intermolecular interaction and potentially promote adhesion of the polymers to heteropolyacids in the composite membrane or to electrodes in order to improve the quality of membrane electrolyte assemblies (MEA)s.